Media in general tends to have magic written in archaic, usually dead, languages. In my world, magic is best described as fickle; most known spells are "iffy" with many having unintended consequences. For example, a spell to cure a lame limb might also turn it ghostly pale or a spell to cure one's cow might cause it to also grow another tail.

The spells themselves are written on tablets, scrolls, etc., passed down from generation to generation within the 'Mage Families'- people who bought spells in the market, happened upon an ancient library or wizard's tower, or accumulated them over the years.

While new copies of spells can be made, only a few select spells are seen as worth copying due to them having been used enough times to have a clear outcome and side effects. However, the spells can only work if they are transcribed exactly as written, which means writing in the ancient "Magic Languages".

What would be a good in-universe justification for the "Magic Languages" to be a series of dead languages while contemporary languages are unable to have any ability to make spells work?

Note: by dead language I mean no one speaks it as their native language, and the Mages themselves have a very minimal understanding of it.

In this world dreaming is a very important part of the Theology so tying in dreaming would be nice, but isn't necessary.

Brand new spells are very, very, very hard to create with only a few being made every hundred years or so. However, finding new spells is rather common.

$\begingroup$How are spells triggered? Do you have to read the words aloud, or is writing them down enough to trigger the spell automatically?$\endgroup$
– F1KrazyJul 1 at 23:02

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$\begingroup$Your mention that brand new spells are hard to create makes me wonder if that would be an acceptable answer in itself? Back when most spells were created, English didn't exist and no one in present time knows the fundamentals well enough to create a spell in English.$\endgroup$
– JollyJokerJul 2 at 7:17

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$\begingroup$@JollyJoker true that could be an answer. I mainly put that in to prevent answers like "just write them in english", but also as balancing in my world.$\endgroup$
– Celestial Dragon EmperorJul 2 at 11:55

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$\begingroup$Wouldn't the fact that the language inacts magic would make it dead promptly? If you can't write a loveletter, bill or diary in a language without casting dozens of spells with effects and side effects, you won't be writing in that language anymore. You also don't want to speak that language since dreams are important in your world and if you dream in that language, that might cast powerfull and potentially draining spells that can kill you or destroy everything around you. Possible plot tool: a powerful wizard that is able to willingly uses dream magic by lucid dreaming.$\endgroup$
– hajefJul 2 at 15:52

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$\begingroup$Perhaps it's the other way round: The language is dead because it can be used for spells - and there is that one strange looping spell that kills all active speakers of the langugae it is written in ...$\endgroup$
– Hagen von EitzenJul 2 at 21:57

48 Answers
48

Immutability

A language is dead if it is no longer in current use. One of the effects of nobody using the language anymore is that it doesn't change.

Consider latin, for example. A tersorium is a collective hygiene device. There are no more romans around, so you can be sure that unless there is a revival, one hundred years from now a tersorium will still be a stick you use to clean your butt.

English, though, is the current lingua franca of the modern world. It changes all the time. It used to be that:

nice meant stupid;

gay meant joyful;

boners were honest mistakes;

thongs were sandals;

You totally don't want to botch a spell because something changed meaning. Imagine trying to solve a drought on a poor third world country and then all of a sudden datacenters are falling from the skies on the poor villagers.

All magic is performed by genies

You may think that you have mastered all secrets arcane and ineffable, and that you are able to bend and rend the very fabric of reality. In reality, magic is a lot like programming. The computers are invisible astral creatures that have been around for a long, long time - and they were created by the very same people whose language is now dead. That's why they only understand sentences in those languages.

$\begingroup$There's also geographic mutability in living languages. A thong is still a sandal in Australia.$\endgroup$
– CadenceJul 1 at 23:48

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$\begingroup$Imagine trying to solve a drought on a poor third world country and then all of a sudden datacenters are falling from the skies on the poor villagers. I think that's an ... interesting side-effect.$\endgroup$
– Tobias F.Jul 2 at 6:13

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$\begingroup$A thong is the strap keeping the sandals of this type attached, resp. the strap connecting the small pieces of fabric in the garment. Both other meanings actually are abbreviations.. Meanings can also change in a dead language because it is still interpreted by the current users (spellcasters). This will not happen in the "programming spells" world, which is a great idea.$\endgroup$
– ChieronJul 2 at 8:54

Your Dead Language is a Programming Language

The speech of modern folk is merely meant to communicate with one another, but the ancient tongue is a programming language that directly shapes reality. An accidental slip of the tongue could be catastrophic for both the utterer and those nearby. Therefore even in the days when it was well known, it was seldom spoke. Each successive generation learned less and less of the ancient tongue until we arrive in the present wherein the more esteemed mage families have managed to preserve different bits and pieces which have come to be associated with them. Additionally, much of the written works of the ancient ones were scribed in their signature language, but the surviving works are treated with caution as there seems to be no one who fully comprehends what the utterance of those words would invoke.

As a bonus, you asked if dreaming could somehow be connected to the magic here. There's actually an excellent opportunity for that. See, dreamers enter a non-material plane during their slumber wherein consequences are typically less severe (yet consequences still exist, often in another form). Practicing magic here requires practice in attaining more lucid dreams as well as remembering the words to be practiced in the first place. The ancient ones understood this and that is how the language was originally kept alive, but that practice has long since died out. Now, few people are even aware of that possibility and fewer yet are capable of such a task.

$\begingroup$There's an interesting variation on this theme in Diane Duane's "Young Wizards" series, where the "Speech" is the direct symbology and language used when the Powers created and described the worlds. You don't mess with it, because merely describing something in the speech in a way that it is other than it is has a certain amount of power to change it depending on various factors (a major plot point.)$\endgroup$
– WenlockeJul 2 at 8:27

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$\begingroup$Charles Stross uses exactly this approach in the Laundry Files. "Old Enochian" is basically a Turing-complete scripting language. The primary reason in the Laundryverse for people to be careful with spoken spells is that there exist magical predators attracted by magic users, not dissimilar to the Warhammer "warp demons" idea. Unlike Warhammer, predators come in all sizes, so if you do the magic yourself (instead of writing it down, say) then you end up with CJD-like symptoms as a result of microscopic magical critters eating a little bit of your brain each time.$\endgroup$
– GrahamJul 2 at 9:50

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$\begingroup$More than a Programming Language, you should say a Formal Language I think. This notion oppose to natural language by the lack of ambiguity, with the use of a strong and mandatory syntax. Mathematic notation are another example of formal language. You don't want ambiguity when your code run, or while solving equations, the same is really true with magic, especially if you want a controlled fire, or a precise tumor removal.$\endgroup$
– CailloumaxJul 2 at 14:47

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$\begingroup$Dragon Age: Inquisition has a very similar plot point: All magic originates from the Fade, a fundamentally magical reality; the "normal" world is a temporary bubble within the Fade created by temporarily locking in the rules. Ancient elves would visit the Fade in their dreams, and were more powerful for it.$\endgroup$
– GraultJul 3 at 1:51

Because they're the original spells and no translation could ever get it right.

As an example of a holy book, the Torah (the first 5 books of the Bible) is written in Hebrew and will always be written in Hebrew. The same ancient version of Hebrew it was first written in thousands of years ago.

When you create a new Torah scroll, you copy each letter exactly. It must be done by someone not just pious but highly trained. Then it's checked. A single mistake (if it can't be scraped clean off the parchment) means the entire panel is no good and must be started over.

Of course the Torah exists translated into pretty much every language on Earth. The entire Bible does. You can get meaning from it in English, Swahili, etc. But if you're going to study it, if you care about the precise wording, you must study it in Hebrew (not just any Hebrew, but Biblical Hebrew).

While there are some very specific prayers, some magical incantations, and also puns, numerology, and shared roots in the Hebrew, this is a holy text and not a spellbook. Still, sticking with the exact original wording is vital. New prayers can be in any language, but they're usually also in Hebrew, to match the Torah. Usually with phrases taken from the larger Bible. The individual words and phrses have specific meaning that can not be translated 100%. (Note: I'm only talking about Judaism; other religions that have used our Bible have different language uses.)

If real-life holy books need this level of language precision, then a book with magical spells would need it even more so. Some words translate easily (a cow in English is probably exactly the same as a cow in Aramaic) but most have subtle differences. Any difference can doom a spell to failure (or disaster).

$\begingroup$+1 for your main point that "If real-life holy books need this [precision]... a book with magical spells would need it even more"$\endgroup$
– J. Chris ComptonJul 2 at 13:12

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$\begingroup$Even a word like "cow" will have subtle differences stemming from its alternate meanings. (In English, "cow" is a derogatory term for a woman; I have no idea whether Aramaic has the same feature.)$\endgroup$
– BrilliandJul 3 at 21:30

You don't use dead languages in casual conversation. That's what makes them dead, after all. Sure, you might casually drop a smidge of Latin into your diction, but usually only a little and in distinct fixed phrases. We can talk about an ad hoc solution or the like, but we don't inflect or twist these phrases in our day-to-day lives the way that a native speaker might.

This is good, because if your words are magic, they're dangerous. Say I have a fire spell, and I associate it with the plain English word "fire". Then I'm watching a baseball game with my friend and I say "the White Sox pitching is really on fire tonight" (this is a crazy parallel universe after all). If I'm not careful to prevent my spell going off, suddenly that might be more literal. Or it might be my TV that's on fire, or me. These are Bad Things.

It's safer if the keywords for my spells aren't words I'm likely to use... well, ever would be ideal. Rarely used is less ideal but still way better than a word that I use casually on a day-to-day basis. This way the risk of it being accidentally triggered is lessened, and I'm more likely to perceive the need to stop it from happening ahead of time. (If I cast spells in Latin and I'm in Latin class, at least I know enough to read through ahead of time and spot any problems.)

$\begingroup$Harry Dresden's magic doesn't quite work that way. Harry's pseudo-Latin is, linguistically, laughable. What's important is that the words are associated with a specific mindset, which you also have to be in. You have to deliberately chose to say the words in the context of wanting to cast the spell.$\endgroup$
– Keith MorrisonJul 2 at 5:52

$\begingroup$And if you get really angry, and tell someone to "drop dead"...$\endgroup$
– ChronocidalJul 2 at 8:58

LoM deals with more sounds than "modern" languages do. This is because in the case of magic, sound carries with it emotion, intent, expectation, and many other difficult-to-express with definable words than today's languages. The consequence is that rather than having the scant 52 characters of modern Auquoric, LoM has 421 unique sounds, each demanding an unique character.1

LoM is precise. It makes the grammatical gymnastics of modern Fleurt look like a day in the park. Verbs can be conjugated hundreds of ways, nouns can be declined hundreds of ways, verbs and nouns can be converted to verbs and nouns (etc.), and don't even ask about gerunds.2 Half of the reason LoM even still exists is that there are a handful of lawyers who insist that their contracts are written in it to guarantee no misinterpretation. It's so complicated a language that there are a rare handful of people who spend their entire lives simply translating it — and they often translate whole pages into single sentences of Fleurt.3

Not surprisingly, LoM takes a honking long time to say anything. Remember those Ents from The Lord of the Rings? Oh, yeah... they'd be long dead before a poem of any credible length could be verbalized. What's the point of asking your friend if she can hang out at the mall with you if, by the time you're done asking, the mall's closed?

To add insult to injury, magic isn't all that uncommon. And you'd have been surprised by how much damage was caused by little Jimmy Snydectoodle when he was showing off just a bit of a butterfly charm and suddenly sneezed. The little snot has a tendency to be infuriatingly vociferous when he does it, too! Old Lady Henschot still passes out whenever anybody mentions Doc. Crewlon's cow. Not surprisingly, quite a little superstition has grown up over even muttering something that sounds like LoM. You'd be just as surprised how many ways you can express the idea of "if you do this, you'll get the beat-down of your life" with fairies.

The subtleties of LoM are such that it requires the listener to actually concentrate when they're listening to people they don't like very much. Remember: sonically and lexically complex, and long-winded to boot. If you pay enough attention to that pond scum to actually understand what he's saying, he might think you were his friend! To make a long story short, people are intrinsically lazy and will rarely use a complex language if a simpler solution exists.4

It's simply impractical. Have you ever tried to tell your husband what groceries to pick up over the phone in LoM? You could turn your mother-in-law into a newt! Which isn't actually a bad thing, per-se, but "significant others" tend to get significantly bent out of shape over things like that.

Finally — it's lost. Considering how irritatingly complex LoM is, it's little wonder that most of it is lost. There's a handful of books written in it and a primer or two, but nowhere near enough to actually study the language — unless you're one of the lucky few who have been initiated into the School of Magic5 where a few Master Wizards jealously guard the few remaining Tomes of Arcane Knowledge.6

1Hiccups or stuttering is a really bad thing when it comes to spells. It quite literally can mean your client, rather than being fabulous, is instead flatulent.

2I speak Finnish, and Finnish is quite a bit like this. A friend of mine and I once spent a couple of hours calculating how many ways a single noun could be converted grammatically into another word or meaning. I kid you not, we stopped at 250,000. It's a wondrously complex language, and just to slap the rest of humanity in the face, it's absolutely beautiful to listen to. So many vowels!

3If you don't think this is possible, you haven't read anything by Fyodor Dostoyevsky.

4Think about the development of Hiragana, Kanji and Katakana — but for completely insouciant reasons.

5And the School of Lawyers... but they're really closed-lipped about the whole thing. It's amazing what a jealous wizard can do to an attorney's little finger.

6And a good thing, too. Can you imagine what Jimmy Snydectoodle could have done to Doc Crewlon's cow if the little punk actually knew anything about LoM?

$\begingroup$This makes most sense to me. Its not that its a dead language per se, its just not useful for normal communications, so there's no reason for anyone to use it.$\endgroup$
– GrandmasterBJul 2 at 17:13

The more a spell is used, the more reliable it becomes.

At its core, magic is powered by belief. A family spirit can mature into a god as it gains followers, a mundane artifact may acquire magical properties as it grows older and becomes associated with good luck, a mysterious forest grove may become a Place of Power if it is regularly used to commune with the spirit world. The same is true of rituals, whether they involve magic circles, runes, or verbal spells.

Most common spells were created when the now-dead languages were in use, and the more they are used the more people believe in their power - which is what gives them their power in the first place. It isn't that people can't write spells in new languages, but why reinvent the wheel? If the ancient word for fire, combined with a particular wand motion, became the ritual for creating a fireball 3,000 years ago, every time that spell is used it reinforces the belief that the particular ritual has that particular effect and therefore makes it more potent and reliable. Magic being the fickle, unpredictable thing it is, you're going to want the spell's effect to be as reliable as possible.

Spells may be created on occasion, but how much is really new? Most "new" spells are just combinations of old ones and will still benefit from the belief in those older spells. So the ancient language will continue to be used by the magic community long after falling out of common use.

Societies that developed magic eventually destroy themselves, so what is left behind ? The left-overs of their highly developed (but highly dangerous) magic. And what language will that be in : the language they developed.

But need that language be real. In many fields we develop specialist language that becomes almost a language unto itself. The military often have their own ways of communicating for speed and accuracy. Lawyers use Latin quite a bit, as do many aspects of medicine and science. Expressions in specialist uses hardly relate at all to their everyday meanings.

So the old texts and documents that the lost (destroyed ?) race developed their magic in may seem like an old language, but it's possibly more akin to a specialist offshoot of a real language.

But magic comes with a price. As it is developed the forces become greater and the power it yields, both in physical and political terms, becomes enormous. Eventually there will be a war and little left behind. What records survive are mere shadows of what was possible - the equivalent, let's say, of Newton's Laws with the magical discoveries of more sophisticated rules left.

And the societies that are left after the magic powered wars don't encourage magic, so it's hidden away. A cult, a family secret. Eventually the meaning and true scope are lost and all that remains are the dead language.

But societies forget and when they take up magic again, they see only the power and not the danger ...

What would be a good in universe justification for the "Magic Languages" to be a series of dead languages while contemporary languages are unable to have any ability to make spells work?

So in this scenario magic languages hide the "structure" of magic which the ancients understood. The vocabulary is lost and the number of samples (spells) known is too few (and too closely kept and secret) to be used to reform the required language.

And some of those sounds made are e.g. complex names buried in a complex grammar that is context sensitive. Maybe making these sounds hurts and requires extensive training (and a willingness to suffer). And what are the names ? Maybe they're Gods, maybe something worse, maybe not so friendly.

Perhaps the names are whole spells.

In this world dreaming is a very important part of the Theology so tying in dreaming would be nice, but not necessary.

Dreaming, when your mind is open to connect to the magic realms, perhaps. When the ordinary "safeguards" that keep you bound within the walls of common sense are weaker or gone and ... something ... can make contact. But "they" aren't human and don't speak human languages or have human thoughts to share, so what you get in dreams are images which you try to interpret.

Brand new spells are very, very, very hard to create with only a few being made every hundred years or so. However finding new spells is rather common

Sort of a contradiction there. I presume this to mean finding "new" (meaning lost) spells is common, but creating them from scratch is hard.

Let us say we must find the whole spell (name ?) exactly and perform it correctly to connect with the source of the spell's power. Are we activating a machine ? A God ? An angel ? Who knows. We don't know what we're doing and we can't just accidentally come across the names of these ... things. Sometimes, rarely, people make a lucky guess. Maybe, as many early scientists did, they get it right but for the wrong reasons. They mix up words from different spells and, by accident more than design, they hit something useful. But most of it is nonsense.

In my world magic is best described as fickle most known spells are "iffy" with many having unintended consequences. For example: a spell to cure a lame limb might also turn it ghostly pale. Or a spell to cure one's cow might cause it to also grow another tail.

The Terry Pratchett school of magic ! OK, this is applying a cost to using it. But this also plays into the idea that eventually magic kills your society. People are curious and they don't ever seem to stop trying to find out more (at least some of them). Magic may come at a price, but it brings power. Power brings ambition and greed. Eventually a society simply steps into far too dangerous a new "level" of magic and perhaps even the act of discovery is death, not merely to the practitioner, but to the whole race ? Perhaps the spell ("name") you uncover is one that does not want to be found. Magic - it's even more dangerous than Physics. :-)

Magic spells are actually a set of instructions to more-or-less intelligent entities - demons, elementals, or whatever you want to call them. These entities are very long-lived, and not really good at learning new languages. Like humans, there's a period in their "childhood" in which they acquire language natively. After that period ends, language learning is much more difficult for them than for the average human.

The bottom line is that most of these entities learned Latin, classical Greek, or some other dead language, and only understand that language. Obviously, therefore, any communication with them must be in the "dead" language.

As an amusing corrolary, there are some entities who only understand REALLY dead languages, like Etruscan or proto-IndoEuropean. And of course the older an entity, the more powerful. You might find a youngster that understands Shakespearian English, but it'd be outclassed by an Old English-speaking one, and both would be outdone by Latin, Greek, or Egyptian-speakers.

$\begingroup$now I'm getting an idea in my head: maybe this world's version of Prometheus gave the original humans the magic language which turned out to be instructions to some fancy smancy celestial beings.$\endgroup$
– Celestial Dragon EmperorJul 2 at 12:15

Magic is ingrained in the fabric of reality, and reacts to specific spoken commands.

Based on #1, everyone who can speak the words correctly, can do magic.

You do not want to accidentally set people on fire.

Imagine that the common magic language is English, and the word "Fire" actually sets the thing you're looking at on fire. Now a fire breaks out, so you look at your colleague, and shout "Fire! The house is on... oh crap!" You just roasted an innocent person.
By using a language that does not map into anything normally spoken, you avoid the inconvenience of magic triggering at wrong times.
Please educate your children on dangers of speaking nonsense made-up words, as that could hurt or kill someone.

Language drift causes meanings to be lost. An example is comparing the English language of Shakespeare to modern English. In just 500 years there have been subtle shifts in pronunciation, whereby people who have tried to recreate Shakespeare in "original Pronunciation" have rediscovered jokes and puns that Shakespeare had written for audiences to enjoy but which modern audiences have no idea are even there.

Even deeper in the past, we have ancient Greek. Homer's Iliad and Odyssey were composed so far back in the past, the "W" sound had actually vanished from then modern Greek when it was transcribed. We only know this if we are reading or listening to it in Ancient Greek-some lines actually don't rhyme because the sound is missing.

So using very exact original pronunciations and language ensures the meanings that power the spells are preserved, including hidden meanings that (when the spell was composed) might need the understanding of an initiate to decipher.

How well rote learning of spells in a dead language works could be the subject of debate. If you actually don't know, recognize or understand the hidden meanings in a spell, will it actually work? Or is the lack of understanding the real reasons spells seem unpredictable these days?

The Ancient Language was Pulled From the Dream World by the Gods

Like Prometheus, Odin hung himself from a tree (or in some cases sacrificed an eye) to find the runic language in a dream. Thoth invented the language of magic in Egypt, but Thoth did not do such things by himself, rather he worked as the scribe of greater gods, following them on their daily activities, and writing the 42 books that contained all knowledge that humanity required. He then delivered the knowledge either directly, or secretly through his wife (stories vary) to humanity.

It may be that, likewise, the "good" languages for magic have a connection where some hero (or deity) wrested the language from the dream.

I think I'd go the other way and have the explanation for modern language be sequences of sounds and letters that have been discovered to be safe.

You could add in a chaotic "Dark Ages" where people used to accidentally blow heads off and turn limbs into snakes all the time, until 'safe' sounds were discovered. Once a working safe languages quickly spread the globe, and there are only rare samples of the more interesting phrases that survived.

Magic is instantiated by translation

Familiarity breeds contempt. Well, really what happens, is when your brain is familiar with a language, it goes straight from conceptual reification in your head to pronunciation, with little to no delay in between. Casting a spell requires a certain amount of mental gymnastics to solidify the reality altering creation of aspell, and it's easiest to enact these steps when you have to struggle a little with the translation. Basically it's that struggle in the middle of "knowing" and "saying" where the actual magic happens, and the better you know your language, the less struggle happens when you cast the spell.

Eventually, with enough castings, you can learn to embody the struggle rather than focusing on the knowledge or pronunciation part, and some mages can even master casting as spell without speaking the words.

On the flip side a mage that studies too hard can learn a language well enough that it impairs their ability to cast spells in that language leading to a sort of zen like situation where the more you learn a spell, and practice it, it becomes both easier and harder to cast at the same time. You need intent without foreknowledge, a lovely little catch-22 that only the most masterful wizards can overcome.

Words-meaning combinations cause magic to cure with time and usage.

Languages change over time. Not only that but it changes fairly quickly, listen to old english (which is, in the span of human history, still new) and you'll be hard pressed to understand it. Magic flows into the world through our words, and when a new word (with its meaning) is uttered it punches a hole in the barrier between reality and the realm of magic. The more it is used the larger that hole becomes, and there is a minimum threshold that people need to effectively use it in a spell. The commonness of words we speak today are still newborn and fledgling in the magic they can pull, much too small for our uses.

Dead languages though, they had been used for generations. The gap they have made is substantial. A merely old word for fire may cause something to just barely feel warm to the touch. A word for fire that had come to mean heat and life for a hundred generations has gained the traction to combust brilliantly.

Why age over quantity?

When words, spoken not as spells, are uttered they flow through the same hole. It is not the usage of the word that expands this hole, but how long that hole had been used. The tapestry between reality and magic is infinite, so too are our words. As each spell invokes the dead language's meaning into pulling the arcane from that plane so too does it expand that hole ever so slightly. The tear gets infinitesimally bigger, as the fraction of all of time that it had taken up gets a bit bigger.

The older the language, the better, but as language ever evolves so too is there a moving window of dead languages. Finding the oldest languages doesn't grant you insane magical abilities, you still have to have the capacity to pull from a large rift/hole/tear caused by the word. Equally, the words only tend to get to a certain size before the memory of it fades from existence and new words are created to replace it. As time flows ever onward, so too does the shifting language palate of magic.

All words are magic. Speaking a word makes use of some of that magic to convey the meaning to the recipient.

If a wizard speaks the word, all is well. The wizard uses the word's power to convey meaning. At the same time, however, a word spoken by a wizard receives some of the wizard's magic and becomes magical itself - in total, the word gains magic.

Muggles, however, do not have any innate magic. They use the word's magic to convey meaning, and so leave it a bit less magical. Enough muggles using the same word drains it of all its magic until it is about as useful for a spell as lint. (just try it yourself: repeat a word quickly and often enough, and it looses all meaning for a short time)

Ancient languages like Latin have been dead long enough that there are more wizards speaking them than muggles, ergo they are gaining magic once again. However, if you want real power you need to broach into the more obscure languages like Sumerian, ancient Egyptian, or pure magical languages (if they exist)

The lost/dead languages may be the language of otherworldly beings which exist within dreams, or sleeping Gods whose dreams' intermingle with the dreams of mortals. It's not a language humans have ever used commonly, and all existence of it in this world comes sporadically from the brief contact made with these outsiders over the millennia through dreaming. What snippets of language that can be remembered after waking have been written down over the years. This is why there are no extensive writings in this language, why there's no record of a kingdom that once used it, and why no one speaks it and knowledge of it is so limited. The words are of dreams, and as such serve as a conduit from the dreaming world to the physical one. Just as dreams are confusing, unpredictable, and reality bending, the spells' effects too follow that form; hence the "iffy" and unintended consequences they bring.

The dead/ancient languages had a different basis and purpose than the more common modern languages. Modern languages are concerned with meaning, they serve to communicate about the mundane world and things directly experienceable by a person, feelings, ideas, concepts, etc. The ancient languages on the other hand were based on the sound vibrations they produced, they could also convey meaning, but in their essence, when correctly spoken, they "interacted" with reality (as in sound-waves interacting with matter, cymatics, etc).

"In the beginning was the Word, and the Word was with God, and the Word was God." -> for a real world example of the idea.

Some people say that specific languages like Sanskrit have "vibrational" properties. If you ever heard Buddhist Monks chanting you'll also notice (probably) that the sounds "reverberate" (although the words being spoken have a meaning as well).

In a universe I'm currently writing what allows magic is the fact that the atmosphere is permeated by an element that reacts to mental activity (it all happens inside a simulation although in the story that's never disclosed or sufficiently hinted, and the people inside it don't know it either, but that's why it's possible)
In your universe you could make it be that the produced sound along with the right focus, when correctly speaking those languages, interacted with matter.

This would make magic very powerful and dangerous as well - it would involve not only the meaning of the words but also producing the "correct" sounds through your mouth and throat, setting your mind in the right context - usually requiring great deals of concentration - that's why spells could easily fizzle. Not because they're unstable but the correct "pronunciation" of the words, while keeping the right frame of mind, is very hard to do (it could also be a secret among the most powerful mages, that they learned it through trial and error for instance and noticed that different ways of "speaking" the same words would produce different effects and started taking notes on which worked well - still this would be naturally limited, some would die in their experiments and besides oral tradition there's really no way to convey exactly the right way that a sound has to sound).

For the dream part. The fact that the "spells" are based on the vibrational characteristics makes it so that while dreaming the characters could "sense" that kind of sinestesia, between the words, their vibration and the right "context" of their mind for it to happen. This means they wouldn't need to become experts in the whole language to be able to cast a spell they "sensed" in a dream, rather because they experienced all those things (word, "pronunciation" and state of mind) in the dream they would be able to replicate the whole feeling of it - so dreams could inspire intuition on how to cast certain spells without making a character suddenly super boss of everything.

It's not the words themselves that are important, but the electrical impulses in the language center of the brain of the caster. Reading the dead language makes you say the correct set of things, making the correct set of impulses - knowing what the language means is completely unnecessary as long as you know how to pronounce it. Spells could be written in the non-dead languages, but as you mentioned writing new spells is hard as finding the sounds that cause the correct electrical impulses is difficult. This could be combined with some of the other answers suggestions that the dead languages were designed to help in memorizing the correct sound patterns to cause the correct electrical impulses, and also why the spells have other consequences - its hard enough to find a spell that gets a nearly right set of impulses.
You could also have as a cool side effect that a character could discover a spell in a dead language - but not know exactly what it does as they can only translate parts of the spell.

Along a similar line of reasoning to what @JollyJoker suggested, maybe the ancient words were themselves 'enchanted' (much like you enchant ordinary objects, like the talismans you mentioned) by the race of people that happened to spoke the now-dead language to possess certain fundamental 'powers' or harness the power of an 'element'. You make spells by combining the 'enchantments' of the words but 'enchanting' new ones (in whatever language) is a lost art.

In the beginning, there was a word... and all words were powerful. But, people need words to communicate. New words are created, they carry only a meaning, but not a magic. The magic words are tabooed in order to avoid triggering unintended magic.

The language evolves, the magic - not really. The world ends up with a language that is good to talk and has no magic in it.

The magic is alive by itself

Magic isn't so magic as everybody thinks. Maybe magic is produced by an omniscient entity, such God or the weave, or is produced by a collection of minor gods, spirits or demons.

Those creatures hear the calling of wizards and perform the magical effects according to the set of instructions given by the spell incantation.

Those primordial creatures exist from the beginning of time, and only speaks a single (now dead) language. That is why spells use that language, they use the language of the person who made it / who produce its effect.

Maybe it's just a blessed language. During the time of creation, gods made this language and blessed it with magic, hiding power on each word. New, alive languages lack this blessing and so they can't be used to cast spells.

It isn't a language, but a collection of magical sounds and instructions

It's told to be a language, but that isn't true. These words, or for being exactly sounds and written drawings had inherent magical properties. They're used to channel, unleash and command hidden sources of power from the very core of reality reprogramming the very laws of physic. Through a deep, large and precise research, wizards from all times have been collecting and cataloguing those sounds and symbols (or commands), carefully tailoring and crafting spell's incantations (or scripts).

No one knows the meaning of those ancient words if they have one. The only current documentation we have are the spells itself, the rest was lost during the ages.

It's inherently magical

Similar to both ideas above. Speaking in your every day with a magical language is extremely dangerous. You wouldn't like to accidentally unleash the anger of a thousand of demons or to change the fabric of reality with a single "Hi".

It isn't an spoken language but a mathematical or programming language

Magic can't be done using such ambiguous languages as we use to speak. An incantation must always mean the same no matter the age or context. That is why the first wizards made an artificial language, a perfect template to make blueprints for new spells. Such annotation language is specifically designed to make spells, and so it's unspeakable for everyday uses.

For example, programming languages are -like the name suggest- languages, but they are used to make programs, not to speak. You can make a program in Python or C#, but not speak your every day on it.

Magic languages have the same effect, they are used to describe the behaviour of spells, not to talk. That is why no one can translate magic languages to normal speech and retain its power. You can turn a C# script into pseudocode or even plain English, but the compiler won't compile, as the spell won't be casted.

It requires an exact structure, pattern and meaning

Compare:

El árbol volador es verde y esta junto al río. Esta lloviendo.

With:

The flying tree is green and it's next to the river. It's raining.

Look the differences, "árbol volador" is translated as "flying tree" instead of "tree flying". "El árbol" becomes "the tree", which lose the grammar genre. Both "es" and "esta" are turned into "is", which is context dependent. "It's raining" use a neutral pronoun, which doesn't exist on Spanish.
All this slightly changes in meaning produce a hugely side effect on spells, which is too dangerous.

It helps mages to maintain focus

Reciting a completely memorized incantation helps to maintain concentration. Reciting something in a language you know allows you to understand it and become distracted. Incantations don't mean anything, they are just techniques to help spellcasters to focus and channel energies. If you accidentally think in anything else, the spell may go wrong. That also explains why each spell has so different phrases, to avoid commit mistakes.

It requires a language that can't evolve

Scientific use Latin to make scientific terms because it's a dead language. Since no society uses it on the present day, the language will never change. No collocations will be made not words will change their meaning.

If you use a language which words changes over time (i.e: Any non-dead language) effects of spells will also change over time, which is something that everybody wants to avoid. Maybe the hotness aura of yesterday produces an insanely hot aura which burned everything, while today it produces an aura of extreme beauty.

Why would “dead languages” be the only languages that spells could be written in?

Because they are unchangeable, reach back towards the origins of things and are sacred for those who pronounce such languages.

We see this in languages are truly considered dead languages in the real world.

A sacred language, "holy language" (in religious context) or liturgical language is any language that is cultivated and used primarily in religious service or for other religious reasons by people who speak another, primary language in their daily life.

Pali, Sanskrit, Chinese, and Tibetan are the main sacred languages of Buddhism.

Christian rites, rituals, and ceremonies are not celebrated in one single sacred language. The Churches which trace their origin to the Apostles continued to use the standard languages of the first few centuries AD.

Hinduism is traditionally considered to have Sanskrit as its principal liturgical language. Sanskrit is the language of the Vedas, Bhagavadgita, Puranas like Bhagavatam, the Upanishads, the Hindu epics like Ramayana and Mahabharata and various other liturgical texts such as the Sahasranama, Chamakam and Rudram.

Sanskrit is also the tongue of most Hindu rituals. It is an Indo-Aryan language and therefore a member of the Indo-European language family. It therefore has some similarities with Greek and Latin, as well as with many vernacular languages of Europe and south Asia. Like Latin and Greek, it also has secular literature along with its religious canon. Most Hindu theologians of later centuries continued to prefer to write in Sanskrit even when it was no longer spoken as a day-to-day language.

While Sanskrit has often been associated with Brahmanism, it remains as the only liturgical link language which connects the different strains of Hinduism that are present across India. The de facto position that Sanskrit enjoyed, as the principal language of Hinduism, enabled its survival not only in India but also in other areas where Hinduism thrived like South East Asia. Apart from Sanskrit, several Hindu spiritual works were composed in the various regional languages of India such as Hindi, Assamese, Bengali, Odia, Maithili, Telugu, Tamil, Kannada, Malayalam, Marathi and Tulu.

The core of the Hebrew Bible is written in Biblical Hebrew, referred to by some Jews as Lashon Hakodesh (לשון הקודש, "Language of Holiness"). Hebrew (and in the case of a few texts such as the Kaddish, Aramaic) remains the traditional language of Jewish religious services, although its usage today varies by denomination: Orthodox services are almost entirely in Hebrew, Reform services make more use of the national language and only use Hebrew for a few prayers and hymns, and Conservative services usually fall somewhere in between. Rabbinic Hebrew and Aramaic are used extensively by the Orthodox for writing religious texts. - Sacred Languages

Sacred languages seem to all be extinct languages to say the least including the Ancient Greek of the ancient gods of Olympia and the Old Norse and the culture of the Runes!

Dead languages convey something of mystery and the unknown from which most of the population understands very little. For this reasons spells pronounced in such languages demand respect.

Magic is not only required to be spoken, it needs to be heard. A supernatural being understands every established language (or there is one being for every language), but it hears them all at the same time, hence all the voices casting spells in english cause none of the spells to be distinguished from everyday chatter. There is a hard limit on how many different voices can be heard and acted on.

As a result only spells in dead and almost dead languages will result in a spell.

Two models of 'where spells come from'

An interesting question to ask of the collectively imagined meme of a spell or magical phrase is 'where did it come from?'. Why, for example, would a spell use human sounds? Why would it be made of the same phonemes we use in our speech? Why are all the magical words a bit like normal words, but not the same?

I present two models, which might help with back story for this (and for a well-known book and movie series). The first is that spells are programmed in by master witches and wizards, to be shortcuts for others. The second is that they form a specific magical picture in the mind of the person casting, which ties to the universe's magical field.

A magical universal field

Consider a spell where we have to hold a particular type of onion and utter the phrase 'Alliux!' to make someone we are picturing cry. In this model, the word itself would be essentially meaningless, but its purpose would be to recreate that particular thought from when we learnt this spell, for which the onion and the oniony phrase derived from allium (the family to which onions belong) serve as a mnemonic.

This model would support the idea that you must cast the spell very carefully, that it requires practise, and that ideally someone taught it to you well so that you can recall the state of mind exactly. It would also require the person casting it to be magical, as the spell word by itself is arbitrary.

Such a model might tie into dreams quite nicely; you might recreate some of those magical images in your sleep and accidentally cast them if you are having a fever dream, or are someone who sleepwalks.

Magic as a programming language with shortcuts

In this model, there is essentially a magical field accessed via either a very magical person or creature or via a spell they have created. In this model, the spell word is very important, because it represents the way to access the shortcut. The spell creator might have made it a global spell, or spells could gradually diffuse out from a specific place.

This would make it easy for non-magical people to invoke a spell, as long as they know the word. However, incorrectly invoking it if you aren't careful about the pronunciation, or if the invocation is complicated by requiring input parameters which are hard to simultaneously provide (e.g. it uses the thing you're holding or the name you think of, or any other contextual or mental parameter), then you may invoke it wrongly.

The best spells (like the best code) would fail gracefully if they detect incorrect inputs, or require a validation step (you cast the spell and then do something to confirm it, or an enchanted vision appears to ask you). The worst spells would just take your dodgy input and carry on.

This model has an excellent explanation for the words and language used for invocation; the invocation was chosen by the spell creator, and must not be accidentally spoken. So old spells would use old languages, or words similar to those in old languages. And foreign spells would use similarly foreign sounds. Mixing with people from a different heritage might permit you to learn their language, and some of their spells, if you could only make that glottal sound.

In programming, one of the difficulties is invoking some code using an object you made elsewhere, i.e. combining one codebase with another. Even when very careful, this can cause lots of unintended side effects; the object doesn't have the expected properties or isn't set up in the way the code expected, so the code ends up in an unexpected state. This could be a useful mechanism for accidentally making cursed objects; an ancient Arabian oil lamp being used to invoke a European storage spell accidentally produces a one way tardis which sucks in anything that touches the spout until rubbed by a worthy child.

Another particularly egregious problem could be that someone knows a privilege-escalation spell, and keeps making undisciplined people into spell-creators. They in turn keep making awful spells, polluting the vocabulary of normal speech with accidental invocations. People who lived near such a historically cursed place would have a huge range of shibboleths and unutterables, whole sound groups to be avoided for fear of unleashing irritating spells.

The best spells would in fact be hard to conjure, requiring very specific invocation with very specific objects, to ensure they were not cast accidentally, and to make sure the parameters are indentifiable. This would give rise to things like potions in which particular meaningful items like a lock of hair are dropped at particular points; this is both safeguarding the invocation and identifying the target of the spell.

All languages are fundamentally capable of being used for magic, even unwittingly. But the effect strongly depends on the particular language because of the other two rules.

There is a separate global "mana pool" for each language. Mana is slowly, but completely regenerated every day, and every spell cast on any given day in a particular language has to share the same pool. Therefore, for each language, the more spells cast on any given day, the weaker every spell will be.

The capacity of these mana pools grows a teeny-weeny bit each time that language is used. This means modern languages which have been in use for only a few hundred years have a smaller total mana pool compared to an ancient dead language that has seen a few thousand years of usage.

As a corollary, casting spells in living languages is also possible in theory, but since on any given day millions of people use the words for "fire", "ice", "light", etc., each usage results in a non-measurable change in the world. But when the ancient, dead language is used to say "fire", there is a non-negligible chance that this is the only use of that word on that day and thus would bring a sizable portion of the substantially larger mana pool into the real world to manifest as fire.

As one could see, this would put a limit on the number of capable magic users, as if everybody started using dead language magic to do everyday tasks, the pool of mana assigned to the dead language would be split across much more usage, netting much smaller results for each use. Which would mean people will notice that this "so called magic" is utterly useless and would promptly abandon this art, only to indirectly cause its renaissance in the hands of its faithful keepers.

This is of course why those who know the dead language of magic are guarding their secrets and only sharing them with their carefully vetted apprentices. This also explains why extremely powerful spells are so rare, but the causation is exactly the opposite: those spells (or rather words of a forgotten language) are powerful because they are rarely used.

The spoken words are ritual components

Just like the animal blood, sigils on the ground, feather of a crow killed by drowning, the spoken words are ritual components that need to be used for the spell. No one understands exactly why those specific words are necessary, but they do know that replacing any of the words with other words of equivalent meaning, even in the original language, will have unpredictable side effects. (In-universe, it could very well be that the words, along with other ritual components, were revealed to the original spell maker in a dream.)

In a nutshell, the human meaning of the words is likely irrelevant, what matters is that the right sounds are uttered.

Magic requires a more mathematical style of language; modern languages lack the precision and structure required.

Languages evolve to be practical; homonyms appear, shorthand and slang usually creep in overtime, misunderstandings and ambiguity are common place in speech.
Magic is a complex practice, it requires careful balancing and fine manipulation of forces that are not even perceptible by normal means.

Spells are like equations in physics, detailed highly codified descriptions of actual physical (or in this case metaphysical) forces that usually include an effect and what factors are involved and how they relate to each other. The lack of precision and potential for ambiguity in everyday language could be disastrous.

We use mathematics as the language of physics for its rules and logical structure, lack of ambiguity as well as the detail that can be expression in a concise way, we don't tend to use everyday language.

In the case of your world, an ancient civilization created and used a language with the same properties for expressing and describing magical forces, that mathematics has for expressing and describing physical forces, it has technical terms and linguistic rules which are capable of handling ideas and concepts that aren't even expressible in everyday language (because they describe concepts and forces outside the purely physical world).
In fact you could also say that even this highly structured lingo isn't one hundred percent effective, tiny mistakes still creep into the formulae (rounding errors :D) and this explains the unexpected side effects of most spells. You could even have it that the actual technique of working magic is not that difficult, magical forces respond readily to the mind, however most people lack the clarity of thought and precision to produce any kind of effect by creating order out of the chaotic background interplay of forces.

This conception of a magical language as analogous to mathematics means that spell books are essentially like mathematical proofs, the squiggles and sigils are formulae, the reason for old and powerful wizards having book filled towers (instead of just one book with spells) is because the theory and formulas are complex and hard to understand, reference works are essential to derive new formulas and spells.

As for why the language is dead, a magical version of the Fermi paradox, a culture that develops a sophisticated powerful system of magic, has a chance of wiping itself out (wizard nuclear wars, accidental portal opening letting elder abominations in, very powerful spell goes very wrong and causes a catacylsm which wipes the society out).
These are are all standard tropes of ancient magical societies.

It's not enough that spells are written in obscure languages. They are written in specific dialects of obscure languages and each college and organization uses their own unique variation.

So for example:

Grabbing a spell book from a college library is worthless unless you have spent time as a student at that college learning their specific language, which also gives them time to indoctrinate you into their philosophy about how magic should be used.

If you are a member of an organization that just spent 20 years researching the ultimate spell to eliminate your rival organization, there is no way for the rival org to steal the scroll and use it against you unless they can find a traitor from your org who can read it for them.

Your world is basically some kind of simulation and the ancient languages are the programming languages the simulation is written in. Mages can use them to 'reprogramm' the world to some extend. This gives some explanation for the unpredictable effects of magic: have you ever tried to change some really, really big software, without any documentation of or knowledge about 99% of the code base? something going wrong is the most likely outcome. It might also provide some explanation as to why a specific language might be better suited to a specific task than another.

Your world might have been created by the gods, who created the progenitor races and imbued their languages with the power to make small alterations to creation. Later languages did not receive this blessing, so everyone has to use the existing magic languages. They became dead when the progenitors died out. I feel like this could go well with dreaming, like 'some progenitor ghosts are contacting gifted individuals in their dreams and reveal their language to them'.

Actually, spells can be written in other languages, but it's hard to get the translation right, so hard that it's easier to just learn the old languages. Why take avoidable risks if you can go with a proven and tested method that's not hard to implement? People have been learning other languages for ages.

The magic language is a written language only and it's the symbols that hold the magic, not the 'language' itself. The language is basically just the syntax used to combine these symbols and every symbol is something akin to a natural constant.

You're looking at it backwards. It's not that magic uses dead languages, it's that any language capable of being used for magic is destined to fall from common usage.

Why? You said it yourself: the magic is full of unpredictable side-effects. Some of those might be acceptable if the spell's upside is needed bad enough and the risks are well understood, but that's most certainly not a quality that you want in something connected to your day-to-day language. I'm not talking about the intentional malicious use of magic (a different problem entirely). I'm talking about the ability to accidentally stumble across a spell during normal use of the language, and for that spell to have unpredictable/undesirable side-effects. The last thing you want is for some poet to write a sonnet extolling the virtues of the grilled-cheese sandwich, accidentally stumble across a sequence of words that conjures up a grilled-cheese sandwich, and then *poof* the moon is now actually made of green cheese.

Your population would quickly adopt a language with less liability, and leave the magic to the experts. When a spell is discovered, the words associated with that spell would be reserved for magic use and new, "safer" words would be invented to replace them. The language would slowly drift until it was no longer recognizable as the ancient magical language. What your people now speak is essentially a constructed language, carefully evolved to eliminate anything that might inadvertently invoke something magical.

Thank you for your interest in this question.
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